The invention relates to the controlling of an incident optical beam for reading a track of information on a dynamic medium, and more particularly, to the control and determination of the positioning error of the incident beam with respect to the track. The invention applies advantageously but not limitingly to digital discs, especially those known as compact discs, e.g. a Read Only Memory Compact Disc (CDROM), and most particularly to multifunction digital discs such as a Digital Versatile Disc (DVD), storing image data in a compressed manner.
A digital disc includes a single spiral track whose relief is representative of the binary information stored on the track of the disc. The track of the disc is illuminated by an incident optical beam, for example a laser spot, and several photodetectors, for example four, detect the reflection of the light beam on the disc. The optical pickup formed by the photodetectors then delivers four respective elementary signals, as well as an overall signal, or useful signal, equal to the sum of the four elementary signals, from which the binary information read on the track is extracted.
The directing or slaving of the optical beam to the track of the rotating dynamic disc is performed exclusively on the basis of the four elementary signals delivered by the photodetectors. More precisely, the signals are summed in pairs so as to form two signals which are equalized in an analog equalizer before being shaped, by comparison with a threshold, in two comparators. The two signals thus shaped are mutually phase shifted if the laser spot is not situated on the track. The phase difference between these two signals is then detected, which phase difference corresponds to the positioning error of the beam with respect to the track. This positioning error is then used conventionally in a servo-control loop to modify the incident optical system and direct the optical beam back to the track.
Such a prior art device includes a considerable number of analog components, which add to the size of the device. Moreover, as the technology advances, the modification and production of new components of the device require considerable design and production time.
An object of the invention is to provide a solution to these problems and is intended in particular to improve the accuracy in the determination of the positioning error, while avoiding the use of analog equalizers.
The invention also makes it possible to detect misalignments or local phase shifts between the various analog paths for processing the signals delivered by the photodetectors.
The invention uses a different approach from the conventional systems by using the overall signal or useful signal for the determination of the positioning error of the beam with respect to the track. Previously, this useful signal was used only for the extraction of the data. More precisely, the invention provides a process for controlling an incident optical beam for reading a track of a dynamic carrier of information contained in the track.
According to a general characteristic of the invention, the beam reflected by the disc is picked up by an optical pickup comprising several photodetectors (at least two and typically four). The positioning error of the beam with respect to the track is determined from the elementary signals delivered respectively by the photodetectors and from an overall signal containing the information and delivered by the pickup.
According to one embodiment of the invention, two signals are generated from the elementary signals delivered respectively by the photodetectors, which hereinafter are dubbed xe2x80x9csecondary signalsxe2x80x9d, whose mutual phase shift is representative of the positioning error of the beam with respect to the track. The overall signal which is equal to the sum of the elementary signals is also generated. The two secondary signals are sampled and the samples which allow the determination of the mutual phase shift, by using the overall signal, are selected.
More precisely, according to one embodiment, the two secondary signals are sampled on the basis of a sampling clock signal, the transitions of the overall signal with respect to a predetermined threshold are detected using an auxiliary sampling signal obtained from the sampling signal and having a period equal to a fraction of the period of the sampling clock signal. At least the two samples of the two secondary signals which temporally flank a transition of the overall signal and whose levels are closest to the threshold are selected.
The invention is based upon the observation that the overall signal is theoretically always situated between the two secondary signals and in the middle of them. Also, the overall signal, and more particularly its transitions with respect to the predetermined threshold, will serve as phase references. More precisely, by selecting two samples which almost symmetrically flank the transition, it is made certain that these two selected samples do indeed belong to the two secondary signals, and that the calculation of the phase shift on the basis of these two selected samples will lead to a correct estimation of the positioning error.
Also, the use of this overall signal for the determination of the positioning error makes it possible to avoid using analog equalizers as in the prior art. Specifically, the analog equalizers were intended to equalize the levels of the signals by amplifying levels or amplitudes which are too low. Also, the use of the overall signal as means of selection allows errorless selection of the samples corresponding to even low secondary signal levels.
It would be possible to calculate the mutual phase shift of the two secondary signals solely on the basis of the two selected samples, especially in the case where the sampling frequency is high. This being so, in order to further increase the accuracy of determination of this phase shift, and hence of the positioning error, it is particularly advantageous to select more than one sample for each secondary signal, and to do so, especially if the sampling frequency is relatively low.
More precisely, according to one embodiment of the invention, for each secondary signal, at least two samples are selected which are situated on either side of the predetermined threshold and are representative for each secondary signal of a crossing of the threshold by the secondary signal in the same direction as the transition of the overall signal. An interpolation, for example a linear interpolation, is then performed between the two selected samples of each secondary signal so as to determine, for each secondary signal, a calculated sample whose level corresponds to the threshold. The positioning error is then determined from the two calculated samples.
The person skilled in the art will readily be able to choose one of these alternative embodiments in view of the particular application and in view of the accuracy required for the calculation of the phase shift.
An object of the invention is also to provide a device for controlling an incident optical beam for reading a track of information stored on a dynamic medium.
According to a general characteristic of the invention, this device includes a pickup for picking up the beam reflected by the disc and which includes several photodetectors. The device also includes a controller for determining the positioning error of the beam with respect to the track from the elementary signals delivered respectively by the photodetectors and from the overall signal containing the information and delivered by the pickup.
According to one embodiment of the invention, the controller includes a preprocessor for generating, from the elementary signals delivered respectively by the photodetectors, two secondary signals whose mutual phase shift is representative of the positioning error of the beam with respect to the track, as well as the overall signal equal to the sum of the elementary signals. The controller also includes a processor having a sampler for sampling the two secondary signals, a selector for selecting the samples which allow the determination of the mutual phase shift, by using the overall signal, and a calculator for determining the mutual phase shift from the selected samples.
The controller advantageously comprises a generator, for example a quartz, for generating a sampling clock signal, while the sampler comprises two analog/digital converters able to sample the two secondary signals on the basis of the sampling clock signal. The processor advantageously comprises a digital phase-lock loop for receiving the overall signal and the sampling signal and for detecting the transitions of the overall signal with respect to a predetermined threshold. The selector is then able to select at least the two samples of the two secondary signals which temporally flank a transition of the overall signal and whose levels are the closest to the threshold.
According to another embodiment of the invention, the selector is able to select, for each secondary signal, at least two samples situated on either side of the predetermined threshold and representative for each secondary signal of a crossing of the threshold by the secondary signal in the same direction as the transition of the overall signal. The processor then comprises an interpolator for performing an interpolation, for example a linear or non-linear interpolation, between the two selected samples of each secondary signal so as to determine, for each secondary signal, a calculated sample whose level corresponds to the threshold. The calculator determines the positioning error from the two calculated samples.
Finally, it is preferable for the processor to comprise a low-pass or median interpolator filter linked to the output of the calculator, thus making it possible to interpolate a positioning error with respect to two positioning errors actually calculated, when this calculation is not possible, for example, when the two samples of the secondary signals intended a priori for the calculation of the phase shift error do not in fact correspond to two samples making it possible to obtain the positioning error without error.